This application requests funds to maintain and develop these unique colonies of hereditary athymic rats and athymic-asplenic mice so as to perpetuate the mutations and make available experimental models which are of considerable importance to immunologists and hematologists. The following studies will be performed in athymic rats: 1) breeding studies will seek critical data on the mode of inheritance, penetrance, and expressivity of the gene as well as on the fetal and newborn mortality produced by the mutation, 2) hematologic studies with particular attention to the circulating lymphocyte number, 3) analysis of the immunocompetence by determining the level of humoral and cellular antibodies after immunization with antigens which reacts specifically with thymus derived cells and those which do not involve thymus cells. Cell mediated immunity will also be studied using ear allografts, and 4) histologic examination of the lymphopoietic tissue including sections treated with anti-thymus derived cell serum (anti-theta serum) to determine the lymphopoietic territories devoid of thymus cells in athymic rats. The long-term goal is to develop, characterize, and make available a colony of rats with hereditary athymia and a colony of mice with hereditary athymia-asplenia for further immunologic studies and transplantation experiments.